A Novel Mechanism for Intermediate Intra-Slab Earthquakes: Crustal Dense Block Controls in the 2020–2024 San Pedro Seismic Sequence, Chile

This study investigates the 2020 (Mw 6.8) and 2024 (Mw 7.3) intermediate intra-slab earthquakes beneath Chile's Salar de Atacama basin, revealing a novel mechanism linking crustal heterogeneity to slab seismicity. Through integrated analysis of GNSS data and seismic observations, we demonstrate how a dense crustal block in the overriding plate modulates stress transfer to the subducting Nazca slab, triggering normal-faulting earthquakes at 100–127 km depth. The earthquakes exhibited contrasting precursory surface deformation patterns—subsidence (2020) versus uplift (2024)—consistent with a viscoelastic rebound cycle driven by stress accumulation and release. Our proposed model highlights the critical role of upper-plate structure in governing intermediate intra-slab earthquakes, complementing traditional dehydration-based mechanisms. These findings advance understanding of earthquake generation in subduction zones and emphasize the need to incorporate crustal architecture into seismic hazard assessments.